Quasiphosphonium intermediates. Part I. Preparation, structure, and nuclear magnetic resonance spectroscopy of triphenyl and trineopentyl phosphite–alkyl halide adducts
Experimental conditions for the preparation of triphenyl phosphite–alkyl halide adducts have been investigated. The long periods of heating necessary for reaction of the higher alkyl halides, especially the chlorides and bromides, leads to decomposition with the formation of halogenobenzenes and diphenyl alkylphosphonates, together in certain cases with halogenophosphites, dihalogenophosphites, and other phosphorus-containing compounds. At room temperature, trineopentyl phosphite reacts readily with iodomethane or bromomethane to afford crystalline Michaelis–Arbuzov intermediates, which decompose in solution to yield neopentyl halides without rearrangement of the alkyl group. 31P N.m.r. spectroscopy shows the adducts to have the phosphonium structure [(RO)3PR′]+X–(R = Ph or neopentyl), the chemical shifts lying in the range –40 to –50 p.p.m. from 85% H3PO4 and being independent of the nature of X (Cl, Br, or I). Molecular weight and conductivity measurements, together with the results of a study of the kinetics of decomposition for trineopentyl phosphite–methyl iodide, are consistent with the presence of ion pairs rather than dissociated ions in chloroform. 1H and 31P N.m.r. chemical shifts occur at slightly lower field for the phosphonium intermediates than for the corresponding diaryl (or dialkyl) alkylphosphonates. Long-range coupling between phosphorus and the α-, β-, and γ-protons is observed, that between phosphorus and the β-protons being greatest.